1. Field of the Invention
This invention relates to a new electrically operated control device and system for an appliance as well as to a method of operating the same.
2. Prior Art Statement
It is known to provide a control system for an appliance wherein the system comprises a power source of alternating electrical current that has a repeating voltage wave cycle and a repeating current wave cycle, load means for using the electrical current to provide an output of the load means for the appliance, relay means having normally open contact means and coil means for closing the contact means only when the coil means is energized, and electrical circuit means for interconnecting the power source to the load means through the contact means only when the contact means are closed.
It is also known in the art to use solid state control means in a control device for an appliance to synchronize the switching of an AC power source to a load means, such load means being an inductive load means, a capacitive load means or a resistive load means. Such synchronous switching is used to minimize the current inrush for the load means.
In particular, inductive loads such as motors, solenoids, power transformers, etc. typically have an inductive as well as resistive current response to an AC power source, such as 120 volts AC 60 Hertz. The inductive response results in the current lagging the voltage by a phase angle up to 90 degrees for a purely inductive load. To minimize inrush current, the voltage is applied to the inductive load when the phase angle of the AC power is 90 degrees and the inductive load current is respectively at its lowest value. This is commonly referred to as Crest-firing where the preferred firing and/or switching is made at the crest of the AC power source which is 90 degrees or the complement 270 degrees phase angle. An example of the practice of this technique is switching power to a magnetron power supply of a microwave oven. Typically a triac is used to apply AC power to the highvoltage power transformer when the AC line is at crest and/or 90 degrees. The peak load current decreases when the firing angle approaches 90 degrees, which is due to the reactive impedance response of the inductive load. In the case of the peak currents at firing angles approaching 0 degrees, they are typically at a maximum level which is limited by the source resistance of the AC supply and the resistive component of the load. Resistive loads, such as a cal-rod heating element used in electric ranges, have a current to voltage phase relationship of 0 degrees. Therefore, minimum current inrush is achieved when the AC power source is at 0 degrees. Synchronous switching of loads at 0 degrees is commonly called zero cross switching.
It is also believed that in large power stations that generate electrical current, certain generating equipment is brought on line at substantially a desired level of the voltage cycle of that line.